Method for making a sensor for monitoring hydrogen concentrations in gases

ABSTRACT

A sensor for monitoring hydrogen concentrations in gases, containing a solid electrolyte body, a detector electrode having a surface which is exposable to a gas to be monitored, and a reference electrode, in which the reference and detector electrodes make contact with the solid electrolyte, and the detector electrode contains a platinum metal or a platinum metal oxide. A method of making the sensor is also disclosed.

This is a division of application Ser. No. 124,326 filed Nov. 23, 1987.

BACKGROUND OF THE INVENTION

The present invention relates to a sensor for monitoring hydrogenconcentrations in gases.

Hydrogen sensors are known. They generally operate with semiconductiveoxides. Their drawbacks are that the semiconductive oxides must beheated to a temperature of several hundred degrees Celsius, that theresponse time of the sensor is very long and that they have a highcross-sensitivity for other gases.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a sensor formonitoring hydrogen concentrations in gases which can be manufacturedvery economically and is distinguished, in particular, by a very shortresponse time for the hydrogen to be detected, connected with the lowestpossible cross-sensitivity for other gases that need not be detected.

The solution of this problem provided by the invention is characterizedin that a solid electrolyte is provided which is in contact with adetector electrode made of one of the platinum metals or their oxidesand exposed to the gas to be monitored and with a reference electrode.

If a hydrogen-containing gas flows around the detector electrode, thehydrogen reacts electrochemically with the sensor. The electricalpotential or change in potential, produced at the electrodes, can beevaluated and further processed by a subsequently connected electronicsystem. The invention thus utilizes the fact that the hydrogen containedin the gas being monitored chemically reacts at the detector electrodeand thus generates an electromotive force in the sensor which producesthe detector signal.

Nasicon, titsicon, khibinskite, wadeite (these compounds are describedin S. Yde Andersen, J. S. Lundsgaard, C. Moller, J. Engell, Solid StateIonics 14 (1984) 73-79; H. von Alpen, M. F. Bell, H. H. Hoefer, S.Schindler, Forschungsbericht T 83-095 (1983), Bundesministerium furForschung und Technologie; and Ph. Colomban, H. Perthuis, G. Valasco,Proc. of the Hindsgave Workshop on Solid State Protonic Conductors forFuel Cells and Sensors, Sept. 5-10, 1987, pp. 377 ff. Jensen, Goodenougheds.) or β-Al₂ O₃ are particularly suitable as solid electrolytes. Thesolid electrolyte includes at least one of these compounds.

One important feature of the invention is that the solid electrolyte andthe electrodes are applied to a substrate in thick-film technology. Withthis technology, the sensor according to the invention can be producedin large numbers and very cost-effectively. However, other manufacturingmethods are also possible and suitable, for example the vapor-depositionprocess.

Preferred is a sensor which is covered and sealed with glass, plastic orsome other suitable material in such a way that at least part of thesurface of the detector electrode remains exposed. The cover need coveronly the top side of the sensor if its underside is covered by thesubstrate.

It is also of advantage for the detector electrode to be formed ofplatinum, palladium or palladium oxide. Palladium has the greatesthydrogen permeability of the platinum metals and thus the sensor becomeshighly selective. Additionally, it is of advantage for the referenceelectrode to be manufactured of materials exhibiting stable sodium ionactivity and the reference electrode to be composed of a sodium tungstenbronze. By using a sodium tungsten bronze, the potential betweenreference electrode and solid electrolyte is kept constant.

The invention will be described in greater detail below with referenceto embodiments which will reveal further important features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a sensor made in accordance withthe invention in a first embodiment in which the electrodes are in aplanar arrangement.

FIG. 2 is a schematic representation of another embodiment of the sensormade in accordance with the invention in which the electrodes arearranged transversely.

FIG. 3 is a graph showing the response in millivolts of a Nasicon H₂sensor at three different temperatures, plotted against the hydrogenconcentration of a hydrogen-nitrogen mixture.

FIG. 4 shows the change in the sensor signal in air during the additionof 2% hydrogen.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electrodes and the solid electrolyte are printed in thick-filmtechnology in several process steps onto a commercially availablesubstrate material 1, e.g. 96% Al₂ O₃, in planar (FIG. 1) or transverse(FIG. 2) arrangement, namely a reference electrode 2, a solidelectrolyte 3 and a detector electrode 4. Electrodes 2 and 4 areprovided with conventional connecting leads or contacts 5. Detectorelectrode 4 may be composed, for example, of platinum, palladium orpalladium oxide. Reference electrode 2 may be produced of materialsexhibiting constant sodium ion activity, for example of a sodiumtungsten bronze. After each printing process, the respective layer issintered or fired or oxidized under defined conditions. In a lastprocess step, after electrodes 2 and 4 have been contacted withconnecting leads 5, solid electrolyte 3 and part of the surface ofdetector electrode 4 may be sealed by a covering 6 of glass or plastic.

An example of this manufacturing process includes: pressing the nasicon,heating at 120° C., 30 minutes in air; pressing of the electronicterminals (for example gold), heating at 950° C., 15 minutes in air;pressing of the platinum electrode, heating at 950° C., 15 minutes inair; pressing the tungsten bronze, heating at 750° C., 15 minutes innitrogen; and pressing the glass cover paste, burning at 700° C., 15minutes in nitrogen.

The cover may seal the reference electrode and part of the solidelectrolyte, leaving the detector electrode uncovered, as in FIG. 1.Another possibility as shown in FIG. 2 is that the cover may sealsubstantially all of the sensor not protected by the substrate, butexposing part of the detector electrode.

The starting materials for the electrodes may be commercially availableplatinum or palladium pastes. A printing paste for the solid electrolyteis obtained if, for example, nasicon electrolyte powder is mixed in thecorrect ratio with a vehicle suitable for thick-film pastes, e.g. ethylcellulose and terpineol. The nasicon electrolyte powder is produced, forexample, by sintering of powder mixtures and subsequent grinding orusing a sol-gel process.

Commercially available cover materials (silicone, lacquers) or printedlayers of commercially available dielectric pastes or cover glass pastesare suitable as covers.

The solid electrolyte of nasicon has the chemical formula

    Na.sub.1+x Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12, where 0≦x≦3.

In tests, x=2.2 and x=2 were used and these tests produced good results.

It can thus be seen that the electrochemical reaction between thehydrogen gas to be detected and the sensor according to the presentinvention causes the latter to react very quickly and to emit a signalvia its connecting lead after a short response time, with such signalthen being processed further, possibly in a subsequently connectedelectronic system likewise produced in thick-film technology. (Highimpedance amplifier, level detector).

The present disclosure relates to the subject matter disclosed by thepatent issued in the Federal Republic of Germany, P 36 39 802.0, on Nov.21st 1986, the entire specification of which is incorporated herein byreference.

It will be understood by those of ordinary skill in the art that theabove description of the present invention is susceptible to variousmodifications, changes and adaptations, which are intended to becomprehended within the meaning and range of equivalents of the appendedclaims.

What is claimed is:
 1. A method for making a sensor for monitoringhydrogen concentrations in gases, comprising the steps of:printing athick film solid electrolyte body comprising at least one of nasicon,titsicon, khibinskite, wadeite and β-Al₂ O₃ onto a substrate; printing athick film detector electrode comprising one of a metal or metal oxideselected from the group consisting of platinum, platinum oxide,palladium, and palladium oxide and having a surface to be exposed to agas to be monitored onto said solid electrolyte body; printing a thickfilm reference electrode comprising sodium tungsten bronze onto saidsolid electrolyte body; and covering said reference electrode with aninsulating material.
 2. The method of claim 1, wherein said referenceelectrode is printed by pressing a sodium tungsten bronze paste andheating said sodium tungsten bronze paste at about 750° C. for about 15minutes in nitrogen.
 3. The method of claim 1, wherein said solidelectrolyte comprises nasicon and said solid electrolyte is printed bypressing nasicon paste and sintering said nasicon paste at about 120° C.for about 30 minutes in air.
 4. The method of claim 1, wherein saidsensor includes an electronic terminal of gold and said electronicterminal is printed by pressing the gold and sintering the gold at about950° C. for about 15 minutes in air.
 5. The method of claim 1, whereinsaid insulating material comprises glass and said insulating material isapplied to said reference electrode by pressing a glass cover paste andsintering said glass cover paste at about 700° C. for about 15 minutesin nitrogen.
 6. The method of claim 1, wherein said electrolyte bodycomprises nasicon.
 7. The method of claim 6, wherein said referenceelectrode comprises sodium tungsten bronze and said reference electrodeis printed by pressing a sodium tungsten bronze paste and sintering saidsodium tungsten bronze paste at about 750° C. for about 15 minutes innitrogen.
 8. The method of claim 1, wherein said electrolyte bodycomprises titsicon.
 9. The method of claim 1, wherein said electrolytebody comprises khibinskite.
 10. The method of claim 1, wherein saidelectrolyte body comprises wadeite.
 11. The method of claim 1, whereinsaid electrolyte body comprises β-Al₂ O₃.